1. Field of the Invention
This invention relates to a single wavelength laser with grating-assisted dielectric waveguide coupler.
2. Brief Description of the Prior Art
Optical communication systems typically employ semiconductor laser sources and glass optical fiber communication channels. There are many configurations of semiconductor lasers including various material compositions and various dimensions of the grown layers that form the active region and an associated optical waveguide in the laser structure. The material composition of the active region determines the wavelength of operation. For example, at lasing wavelengths of about 0.9 xcexcm, the group III-V materials of the ternary compound AlxGa(1-x)As with GaAs quantum wells provide a compact and rugged source of infrared light which can be easily modulated by varying the diode current. Communications systems of this type are discussed in Ser. No. 08/248,937, now issued as U.S. Pat. No. 6,064,783, the contents of which are incorporated herein by reference. Light from a laser can be extracted by abutting an optical fiber thereto in known manner, however, devices fabricated in this manner do not lend themselves to semiconductor fabrication. Lasers can be abutted to optical fibers, however the indices of refraction between optical fibers and semiconductor material are so dissimilar that the amount of coupling is very low, leading to an inefficient device. Furthermore, the alignment of the source with an optical fiber is quite tedious when high coupling efficiency is desired. This mismatch of the light field of the laser and that of the optical fiber also affects the amount of light coupled to the fiber.
In the device described in the above noted application, the direct coupling of the semiconductor laser output into an optical fiber was improved over the prior art by providing a semiconductor laser integrated with a silicon dioxide based waveguide having high efficiency coupling of the laser output into the waveguide by an integrated grating to permit the laser output to be coupled into an optical fiber by butt coupling of the optical fiber to the silicon dioxide based waveguide. The grating, when appropriately designed as discussed in the above noted application, provides a matching of the propagation in the laser with the propagation in the glass. The period of the grating determines the wavelength of that portion of the light in the laser waveguide that will be passed through the grating to the optical fiber. Multiple lasers with different wavelengths could be integrated and their outputs coupled and combined into a single waveguide for wavelength division multiplexed operation. A problem with the device of the above-mentioned application is that the narrow bandwidth of grating assisted directional couplers makes it difficult to match them with an integrated single wavelength laser source whose lasing wavelength must lie within the bandwidth of the coupler. The architecture of grating assisted directional couplers typically consists of two waveguides and an optical grating whose period is dictated by the geometry of the two waveguides. The geometrical properties include the waveguide dimensions as well as the refractive index profiles. Generally, the lasing wavelength is governed by an optical grating that produces the necessary feedback to the laser. Precise machining of both gratings must be made to allow for satisfactory operation.
The above noted problems of the prior art are minimized in accordance with the present invention.
Briefly, there is provided a semiconductor laser, preferably of the type set forth in the above noted copending application, coupled on one side of the anode/cathode structure of the laser to a dielectric waveguide via a grating as in the above noted application. The semiconductor laser is formed from either group III-V compound materials or from group II-VI compound materials (the material system determines the wavelength of operation), and preferably includes various hetero-junctions and thin layers that form quantum well regions. Generally, there are two hetero-junctions that form the boundary between a central, high refractive index (relative to the central region). The light generated from the active region (generally part of the central region) is then confined by the high refractive index layer (central region). The coupling grating is formed at the interface of the laser cladding layer and the cladding layer of the dielectric waveguide. The dielectric waveguide is formed from silicon dioxide based materials and preferably phosphosilicate glass which is deployed p-type having from about 8 to about 10 percent by weight of dopant. The dielectric waveguide is preferably phosphosilicate glass with the dopant being phosphorous. The dielectric waveguide includes a reflector, such as, for example, a mirror at an end region formed by high reflection coating the dielectric waveguide facet thereof to reflect a portion of the light therein back through the grating and into the laser to provide the required feedback of the desired light frequency as determined by the grating while all other frequencies which are not passed through the grating are absorbed by the absorber and are not reflected back into the laser active region. The light fed back through the grating will be at the same wavelength as the light initially transmitted through the grating. It follows that the grating is performing the function of both reflection and wavelength selection. In this manner, the desired lasing wavelength is enhanced by the light being fed back whereas all other light wavelengths are rejected or minimized by being absorbed by the absorber.
In operation, broad band light (stimulated emission) is generated in the laser active region. The generated light from the active region propagates to both the front (right direction) and to the rear (left direction). Light traveling in the left (backward direction) is reflected by a broad band reflector (mirror) that may be located at either the edge of the active region or at a distance from the edge. The backward propagating light beam is reflected back into the active region and produces the total beam that is propagating to the right (forward direction). In the absence of the grating coupler, the light traveling out of the front of the active region continues in the semiconductor waveguide section and enters into an absorbing region. As a result, light is not fed back into the active region and, accordingly, the stimulated emission spectrum will not become narrow (i.e., non lasing condition). In the presence of the grating and an auxiliary waveguide that is parallel to the semiconductor waveguide (co-planar geometry), the broad band light within the laser waveguide is coupled (and filtered) to the auxiliary waveguide. The filtered-out wavelength coupled to the auxiliary waveguide is determined by the grating period, grating depth, relative location, and the geometrical and dielectric characteristics of both the semiconductor and auxiliary waveguide. A reflector, which is formed by a high reflection coating on the dielectric facet in the auxiliary waveguide, such as, for example, a mirror, is provided to reflect a portion of the light in the auxiliary waveguide back into the semiconductor waveguide. Since the light reflected by the mirror in the auxiliary waveguide has the specific wavelength (or narrow-band spectrum that was initially filtered by the grating), the light will naturally couple back to the semiconductor waveguide and, accordingly, be fed back into the laser active region. The result is that the stimulated emission within the laser is enhanced only at the wavelength transmitted through and fed back by the grating. All other wavelengths generated from the active region of the laser are absorbed by the absorber at the terminal region of the laser. p While only a single laser is discussed herein, it should be understood that the device in accordance with the present invention can replace one or all of the devices 410, 420, 430 or 440 in FIG. 4a of the above noted application with the grating in each device being adjusted to the desired wavelength.